FIG. 1 of the appended schematic drawing recalls the main elements which usually constitute an electric power assisted steering system of a vehicle. Such a steering system comprises, on the one hand, a mechanical part comprising a steering wheel 2 connected in rotation to a steering column 3, the end of which remote from the steering wheel 2 carries a steering pinion meshing with a rack 4, mounted slidably in a steering casing 5. The two opposite ends of the rack 4 are respectively connected, via link rods 6 and 7, to the right and left steering wheels (not shown) of the vehicle. The steering system comprises, for assisting the manual force exerted by the driver of the vehicle on the steering wheel 2, an electric assistance motor 8 with two directions of rotation. The output shaft of the electric motor 8 is coupled, via a speed reducer 9 in particular comprising a worm and a tangent wheel or comprising a gear train, to the steering column 3 or to the steering pinion or to the rack 4 directly, so as to transmit a motor torque (optionally also a resisting torque) to the steering system. The electric assistance motor 8 is controlled by an on-board electronic computer 10 which receives and processes various electric signals coming from sensors.
In a customary embodiment, the electronic computer 10 receives in particular an electric signal coming from a torque sensor 11 placed on the steering column 3, and thus measuring the torque exerted by the driver on the steering wheel 2, and also an electric signal coming from a speed sensor 12 of the vehicle.
Based on these various items of information, the electronic computer 10 controls the electric assistance motor 8 by defining at any time an assistance torque or force which may amplify or on the contrary compensate the force applied by the driver on the steering wheel 2, according to predefined “assistance laws”.
It should be noted that some steering systems calculate or measure the instantaneous position of the electric assistance motor 8.
It should also be noted that some motor vehicles are equipped with a steering angle sensor which may be integrated in the steering system, or else placed on the steering column for example. The particular feature of such sensors is their ability to supply absolute angle information which is available as soon as the car starts up. The installation of these sensors is nevertheless penalising in terms of weight, volume and cost.
In order to reduce the disadvantages of these sensors in terms of weight, volume and cost, some current steering systems are equipped with simplified sensors which give a point or “top” signal for certain angular positions of the steering system, in particular upon each revolution of the steering pinion. Such sensors are also sometimes denoted as “index” sensors.
By way of illustration of this prior art, reference is made to the patent document EP 1550839 A1.
It should also be noted that some steering systems have to offer automated steering functions, such as the automatic parking function, or automated steering angle limitation for example. These functions need to know the steering angle in order to operate. Moreover, these functions must be active when the vehicle is at a standstill, in the instant following start-up of the vehicle. At present, steering systems equipped simply with sensors of the “index” type cannot be provided with automated steering functions that are active at a standstill in the instant following start-up of the vehicle, even when the system uses an electric assistance motor, the instantaneous position of which is calculated or measured.